1. Field of the Invention
The present invention relates to a reflective guest-host liquid-crystal display device. More particularly, the present invention relates to technology for improving use efficiency of incident light by incorporating therein a phase shifter and removing a polarizing plate. Still more particularly, the present invention relates to technology for improving display quality by removing wavelength dependence characteristic of a built-in phase shifter in a case in which a color display is made, and also to a structure of a micro-color filter required for color display and a method of manufacturing the same.
2. Description of the Related Art
There are various modes in a liquid-crystal display device. At present, TN or STN modes employing a twist-oriented or supertwist-oriented nematic liquid crystal are most common. However, in respect of operating principles, these modes require a pair of polarizing plates. Since light is absorbed by the polarizing plates, transmittance is low, and a bright display screen cannot be obtained. In addition to these modes, a guest-host mode using a dichroic dye has also been developed. A guest-host mode liquid-crystal display device produces a display by using an anisotropic property of the absorption coefficient of a dichroic dye added to a liquid crystal. If a dichroic dye having a bar-shaped structure is used, since dye molecules have a property to orientate parallel to the liquid-crystal molecules, the orientation direction of the dye also varies if the molecular orientation of the liquid crystal is varied by applying an electric field thereto. Since this dye is made colored or not depending upon the orientation direction, it is possible to switch between a colored state and a colorless state of the liquid-crystal display device by applying a voltage.
FIGS. 6A and 6B show the structure of a Heilmeier-type guest-host liquid-crystal display device; FIG. 6A shows a state in which a voltage is not applied, and FIG. 6B shows a state in which a voltage is applied. This liquid-crystal display device uses a p-type dye and a nematic liquid crystal (N.sub.p liquid crystal) with positive dielectric anisotropy. A p-type dichroic dye has an absorption axis which is nearly parallel to the molecular axis; therefore, it strongly absorbs polarization components Lx parallel to the molecular axis and hardly absorbs polarization component Ly vertical thereto. In the state, shown in FIG. 6A, where no voltage is applied, the polarization components Lx contained in the incident light are strongly absorbed by the p-type dye, and the liquid-crystal display device is made colored. For example, if a black dichroic dye is used, the liquid-crystal display device is colored black. In comparison with this, in the state, shown in FIG. 6B, where a voltage is applied, the N.sub.p liquid crystal having positive dielectric anisotropy is turned on in response to the electric field, and in accordance with this, the p-type dye is aligned in a vertical direction. For this reason, the polarization components Lx are hardly absorbed, and the liquid-crystal display device shows colorlessness. The other polarization components Ly contained in incident light are not absorbed by the dichroic dye regardless of the voltage applied state or the no-voltage applied state. Therefore, in the Heilmeier-type guest-host liquid-crystal display device, one polarizing plate is interposed beforehand so that the other polarization components Ly are removed.
In the guest-host liquid-crystal display device using a nematic liquid crystal, the dichroic dye added as a guest is oriented in the same manner as the nematic liquid crystal. The polarization component parallel to the orientation direction of the liquid crystal is absorbed, but the polarization component perpendicular thereto is not absorbed. Therefore, in order to obtain sufficient contrast, one polarizing plate is disposed at the incidence side of the liquid-crystal display device so that the polarization direction of the incident light coincides with the orientation direction of the liquid crystal. However, if this is done, 50% (in practice, approximately 40%) of the incident light is lost in principle by the polarizing plate and therefore, the display is darkened as in the TN mode. As a technique for reducing this problem, merely removing the polarizing plate causes the on/off ratio of the absorbance to decrease considerably, which is not appropriate, and various improvement measures have been proposed. For example, while the polarizing plate on the incidence side is removed, a structure in which a quarter-wave phase shifter and a reflection plate are mounted at the emergence side has been proposed. In this method, the polarization directions of two polarization components perpendicular to each other are rotated by 90.degree. in the forward and backward paths by means of the quarter-wave plate, and the polarization components are interchanged. Therefore, in the off state (absorbing state), each polarization component is absorbed along either the incidence light path or the reflection light path.
However, in this structure, since a quarter-wave plate and a reflection plate are provided externally, the liquid-crystal display device itself must be a transmissive type. In particular, in order to make possible a high resolution and moving-image display, when an active-matrix-type structure is used, thin-film transistors for driving pixel electrodes are integrated on a substrate; therefore, in the transmissive type, pixel aperture ratio is low, and a substantial portion of the incident light is shut off. Therefore, even if the polarizing plate is removed, the screen of the display apparatus cannot be made remarkably bright.
When the active-matrix-type liquid-crystal display device produces a color display, any one of the three primary colors of red, green and blue is assigned cyclically to each pixel. In order to assign these three primary colors, color filters or the like are used. The color filters selectively permit wavelengths corresponding to the three primary colors assigned to each pixel to be passed through. However, in the case of a color display, if a technique for highlighting a black display by using a quarter-wave phase shifter is employed, the dependence of the phase shifter upon wavelength exerts an adverse influence upon display quality. For this reason, a coloring influence appears during black display when the voltage is off. Further, since the polarization conversion effect of the phase shifter is not uniform over the entire wavelength region, a decrease in contrast occurs.
Furthermore, when the active-matrix-type liquid-crystal display device produces a color display, it is necessary to form a microcolor filter which is plane-divided into the three primary color components of red, green and blue in correspondence with each pixel. The active-matrix-type liquid-crystal display device has a structure such that a drive substrate on which pixel electrodes and thin-film transistors for switching purposes are integrated, and a facing substrate on which counter electrodes are formed, are joined together, and a liquid-crystal layer is held in the spacing therebetween. In a conventional active-matrix-type color liquid-crystal display device, microcolor filters are formed on the facing substrate side. However, in such a structure, when the drive substrate and the facing substrate are bonded together, it is necessary to provide a certain degree of margin in the accuracy of overlapping them, and the pixel aperture ratio is sacrificed by a corresponding amount. In a reflective full-color liquid-crystal display device not using backlight, in order to obtain a bright screen, it is necessary to enlarge the aperture ratio of the pixels as much as possible. However, in a conventional structure in which microcolor filters are provided on the facing substrate, the aperture ratio is limited due to the overlapping accuracy for them.